This invention relates to a stereoscopic imaging system for providing stereoscopic images, and particularly to a stereoscopic imaging system which permits observation of stereoscopic images with less fatigue, without producing an unnatural feeling to the observing eyes and without depending on the position of the observing eyes.
Among many factors which determine whether a person gets a stereoscopic view are the perception of the positional difference of the images in the two eyes due to the difference in the position of the left and right eyes, i.e. the binocular parallax, and the convergence-divergence movements of the eyes, i.e. slightly moving the eyes closer to each other and away from each other when viewing an object. A stereoscopic (three-dimensional) television system has been developed which provides stereoscopic images by generating the above-mentioned factors by means of two television cameras and provides the eyes with the images separately, thereby giving the viewer a perception of viewing the stereoscopic object. As means for providing a person with the images, there are a method in which two monitors are arranged to give separate images to each eye, and a method by which the left and right images are superimposed on one monitor and the images are separated by polarization of light or by an electronic shutter. FIGS. 1A, 1B and 1C show an example in which one monitor is used. In FIG. 1A, the left camera 1 and the right camera 2 result in images in which there is a binocular parallax. A field change-over signal processing unit 3 produces a synthesized video signal 4 made by alternately arranging frames L with frames R from the left and right camera heads 1 and 2. An A/D converter 5 converts this video signal into a digital signal which is stored in a frame memory 6, and a frame number converter 7 re-arranges the digital signal as a video signal 8 provided at a speed twice as fast as before. A D/A converter 9 converts this video signal 8 (FIG. 1B) in digital form into analog form, and shows the analog video signal on a twice-high-speed scanning monitor 10. Liquid crystal glasses 11 open and close liquid crystal shutters synchronized with the frames alternately shown on the twice-high-speed scanning monitor 10 in response to shutter signals 12 and 13 (FIG. 1C) synchronized with the frames produced by the frame number converter 7. Therefore, the left eye sees only the left frames L and the right eye sees only the right frames R. Owing to the effect of the binocular parallax, a person can perceive a stereoscopic image. A method is possible which, instead of the liquid crystal shutters, uses polarized light shutters, placed in front of the monitor screen or a projection lens of a video projector, operates the polarized light shutters by turns in synchronism with the left and right frames, and a person can separate the left and right images and obtain a stereoscopic image by wearing polarizing glasses using polarizing lenses, the polarizing directions of which differ between the left and right lenses.
Conventional stereoscopic vision technoligies are discussed in detail at pages 205 to 223 of Nikkei Electronics No. 444 (Apr. 4, 1988) and at pages 1 to 31 of Seimitsu Kogaku bulletin (the Journal of the Japan Society of Precision Engineering), 54/2/1988.
In the meantime, the relative position of the two camera heads 1, 2 is set in the following two ways (FIGS. 2 and 6). One method is that, as shown in FIG. 2, the two camera heads 1, 2 are arranged with their optical axes oriented in parallel. If an object, from which iamges are obtained is located in front of the two parallel camera heads 1, 2, the images are formed on image-forming planes 17, 18 by lenses 15, 16. In FIG. 2, the sizes and directions of images l and r on the image-forming planes are indicated by leader lines.
The two images l and r are displayed alternately on the monitor screen 19 as shown in FIG. 3, and the images are perceived by a person's eyes 20, 21. The left eye views the image 1 seen from the left hand side, while the right eye views the image r seen from the right hand side. As a result, a synthesized image 22 that can be seen by the binocular parallax is located in front of the monitor screen 19. In FIG. 2, if the object 14 is moved along the intermediate axis between the two camera heads 1, 2 to a point at infinity, the images of the object are formed at the central points of the left and right image-forming planes 17, 18. When these images are shown on the monitor screen 19 of FIG. 3, the point at infinity is located at the center of the monitor screen 19. When the object at the infinite-point moves to left or right, since the rays of light from the object are parallel rays, difference in parallax does not occur between the images l and r. Therefore, the points at infinity are distributed on the monitor screen 19, and normally a stereoscopic image of an object not present at a point at infinity is formed in front of the monitor screen 19. The area between the parallel optical axes 23, 24 of the two camera heads 1, 2 in FIG. 2 corresponds to the area between the optical axes 25, 26 of the eyes in FIG. 3. Therefore, the actual space is represented in condensed form in front of the monitor screen 19, and the breadth is also condensed. In consequence, the images are distorted, so that a stereoscopic image cannot be represented correctly. If various lengths are denoted by symbols shown in FIG. 4, positions k and m of the point p (x,y) on the image-forming planes are expressed by the following equations. ##EQU1## where b is the distance between the centers of the image-forming planes 17, 18, d is the distance from each of the lens centers to the image-forming planes, and s is the size of the image-forming planes. As shown in FIG. 5, if the size of the monitor screen is denoted by S, the distance from the eyes of the observer to the monitor screen by D and the space between the eyes by E, the position of a synthesized stereoscopic image P(X,Y) is expressed as follows. The positions K, M of the left and right rather than the screen size are first shown. ##EQU2##
Therefore, the position of P(X,Y) is given as: ##EQU3##
From Eq. (5), it is understood that owing to the effect of the value of y, X comes closer to zero as y approaches infinity. It is obvious from Eq. (6) that as y approaches infinity, Y comes closer to A. From Eqs. (5), (6), it follows that the actual point p (x,y) is deformed and reappears at the point P(X,Y) of the stereoscopic image, so that a natural feeling of three dimensions cannot be obtained.
Another method of setting the relative position of the camera heads 1, 2 is to converge the optical axes of the cameras as shown in FIG. 6. The image obtained by this method is expressed by using the arrangements in FIGS. 7 and 8. The positions k and m of the point p(x,y) on the image-forming planes are expressed as follows. ##EQU4##
Likewise, from FIG. 8, ##EQU5##
The relation between the monitor screen 19, on which these demensions are shown, and the eyes 20, 21 is shown in FIG. 5 as with the above case. Therefore, ##EQU6##
In consequence, both X and Y, subject to the effect of x and y, are not in a simple proportional relationship. Therefore, the images taken by the left and right camera heads 1, 2 have an image distortion in addition to a transverse difference between them caused by binocular parallax, thus posing difficulty in obtaining a stereoscopic image.
Furthermore, there is another problem. When an object is selected which is in a square shape as seen from the front, an image obtained on the image-forming plane 17 at the left is a trapezoid with the right side tapering down as shown in FIG. 9. On the other hand, the image obtained on the image-forming plane 18 at the right is a trapezoid with the left side tapering down. If these two images are shown superimposed on the monitor screen 19, an image with a binocular parallax results, with the corresponding vertical lines having different lengths. In this case, if the left sides of these squares are taken for example as shown in FIG. 10, the synthesized image 27 is not synthesized correctly because these two sides have different lengths in the vertical direction. To be more specific, it is unnatural to obtain a stereoscopic image by synthesizing images of different sizes into a whole image by the work of the brain, and if this is done, the observer will become fatigued.